1. Field of the Invention
The present invention relates to the technical field of generating droplets and, more particularly to a droplet dispensing device. It further refers to a light source comprising such a droplet dispensing device.
2. Description of Related Art
In different technical fields it is necessary to generate a continuous sequence of droplets of a predetermined size and droplet frequency, which is ejected on a predetermined trajectory. The material of the droplets may for example be a liquid solution or a molten metal. One technical field, where such a droplet dispensing device is used, is a light source emitting extreme ultraviolet or soft x-ray light.
Extreme ultraviolet (EUV) light is electromagnetic radiation with wavelengths between 121 nm and 10 nm, while soft x-rays range from 10 nm to 1 nm. In EUV or soft x-ray sources, a radiation emitting plasma is produced by irradiating a target material. A regenerative solution for delivering target material to the production site comprises a target dispensing device based on liquid droplets. The radiation exciting the target material can be a pulsed laser beam, thus producing a laser produced plasma (LPP). The target delivery must be synchronized with the pulsed laser beam. The radiation is typically collected and directed to an intermediate region for utilization outside of the light source.
The formation and delivery of the target material to the focus of the light collector is closely linked to thermal and fluid management, as well as droplet generation issues of the dispensing device.
A method for creating continuous droplet streams with a vibrator coupled to an injection nozzle is disclosed in H. M. Hertz et al., “Debris free soft x-ray generation using a liquid droplet laser plasma target”, U.S., SPIE, Vol. 2523, pp. 88-93, with application in a soft x-ray light source. Another method, based on magnetic coil vibrators is disclosed in “A continuous droplet source for plasma production with pulse lasers”, U.K., Journal of Physics E: Scientific instruments, Vol. 7, 1974, pp. 715-718.
Other dispensing systems are used in soldering applications, such as the apparatus shown in document U.S. Pat. No. 5,171,360, which produces a stream of metal droplets at a temperature of 200° C. A similar droplet generator developed is disclosed in Rev. Sci. Instr. 58 (1987) 279. Another typical continuous or on-demand soldering jetting device is disclosed in document U.S. Pat. No. 6,224,180. A method and apparatus to generate on-demand droplets by acoustic impulses, including an impulse transmitting device inside liquid metal, is disclosed in document U.S. Pat. No. 5,598,200.
Document EP 0 186 491 discloses an apparatus for producing soft x-rays from generating a plasma source which comprises a low pressure vessel, energy beam means, such as a laser beam, associated with the low pressure vessel for generating and supplying a high energy beam to an impact area inside the low pressure vessel, a liquid target, such as mercury, capable of emitting x-rays when impacted by a high energy beam target supply means associated with the low pressure vessel for supplying the liquid target material to the impact area inside the low pressure vessel, and control means coupled to the energy beam means so that the high energy beam impacts the liquid material target in the impact area of the low pressure vessel. The mercury drops are formed at the end of a fine tube in a vessel by the action of surface tension and are caused to drop by vibration set up by a piezoelectric element, which is connected to the fine tube (see also US 2009/0230326 or US 2010/0090133).
Document WO 2006/093687 discloses an EUV light source plasma source material handling system and method, which may comprise a droplet generator having droplet generator plasma source material reservoir in fluid communication with a droplet formation capillary and maintained within a selected range of temperatures sufficient to keep the plasma source material in a liquid form; a plasma source material supply system having a supply reservoir in fluid communication with the droplet generator plasma source material reservoir and holding at least a replenishing amount of plasma source material in liquid form for transfer to the droplet generator plasma source material reservoir, while the droplet generator is on line; a transfer mechanism transferring liquid plasma source material from the supply reservoir to the droplet generator plasma source material reservoir, while the droplet generator is on line.
The droplet dispensing devices known so far have the disadvantage that the droplet size and trajectory are not precise enough to generate extreme ultra-violet or soft x-ray radiation in a laser-driven light source with improved efficiency.